Explosive device



Jan. 6, 1959 T. A; KIBBY EXPLOSIVE DEVICE Filed June 25, 1954 .2., n..fau/. 2.". \....v..... I.. JWM uw.; .....F ...M

2,86 7,170 Patented Jan. 6, 1959 tice pta

EXPLOSIV E DEVICE Theodore A. Kibby, San rlfome, Venezuela, assignor toGulf Oil Corporation, Pittsburgh, Pa., a corporation of PennsylvaniaThis invention relates to an explosive device which is particularlyadapted for use in completing fluid-producing wells inpoorly-consolidated producing formations and more particularly to such adevice made with explosive cord.

In producing oil, gas, or other fluids from poorly-consolidatedformations such as unconsolidated sands, it is desired to avoid theproduction of sand with the fluids in order to prevent costly shut-downdue to clogging of the well bore and to avoid damage to pumps and otherwell equipment. A number of procedures for sand control have beenemployed including the use of various types of strainers in theproducing formation; for example, wirewrapped slotted pipes, gravelpacks, and the like. The prior art means for sand control have not beenentirely satisfactory. Abrasion caused by sand particles enlarges theopenings in the wire-wrapped slotted pipe and allows the production ofsubstantial quantities of sand which usually accumulates in the wellbore, stopping production, and necessitating an expensive cleanoutprocess. The openings in the pipe are also subject to clogging by sandwith resulting decrease in production. Gravel packs are also subject toclogging. This is due to the fact that it is difficult to place thegravel in the well without forming voids in some places in the pack.Such voids allow shifting of the gravel during production and consequentmovement of sand from the surrounding formation which clogs fluidpassages in the pack and stops or reduces production.

The copending patent application of Abraham I. Teplitz, Serial No.424,862, now abandoned, describes a new .method of completing wells inunconsolidated formations which makes possible the production of fluidsfrom unconsolidated sands without the production of substantialquantities of sand. The new method in general comprises installing in awell drilled in a poorly-consolidated producing formation a tubularcasing coaxial with the bore hole, forming a cementitious sheath in theannular space between the casing and the walls of the poorlyconsolidatedproducing formation, and applying a stress to ,the sheath sufficient toform fractures in the cementitious material. In one modification of thenew method the stress is applied by detonating an explosive within thecasing opposite the poorly-consolidated formation. The small fracturesor cracks formed in the cement allow fluids to lter through the sheathand enter perforations in the well casing while sand is excluded. Thefractured sheath retains its general outline and continues to supportthe surrounding unconsolidated sand formation.

The present invention is directed to an explosive device .which isparticularly adapted for use in the method of completing a welldescribed in said Teplitz application. The explosive device of theinvention in general com- -prises an elongated, inert core member, alongitudinal strand of explosive cord on the outside of the core member,wrappings of explosive cord attached to the core member, means forlowering the device into a well and means operable from ground level fordetonating the Aexplosive cord withinrthe well.

The invention can be described further by referring to the drawing, ofwhich Figure 1 is a diagrammatic view of an explosive device of theinvention in position within the producing zone of an oil well; and

Figure 2 is a diagrammatic sectional View of the producing zone of anoil well which has been completed with the use of a device of theinvention.

Figure 1 shows, by way of example, a well extending through aconsolidated formation 10, an unconsolidated oil sand`11 and into aconsolidated formation 12. The well is cased throughout the producingformation with a tubularV casing 14, which is preferably steel casingpipe commonly used in the art. Figure l shows casing 14 as anunperforated pipe which is to be perforated after it is placedin thewell, as will be describedl in more detail hereinafter. However, ifdesired, the pipe can be perforated before it is placed in the well.Conventional shop-perforated or slotted pipes, screen pies and the likecan be used instead of the unperforated casing shown in Figure l.

The casing 14 is surrounded by a cementitious sheath 15, formedpreferably from a slurry of neat Portland cement. Care is taken toeliminate voids or channels from the cement sheath l5 before it sets,for example, by using scratchers and centralizers in the cementingoperation. After the cement has set, the well is ready for .the phase ofthe completion method in which fractures are formed in the cement sheathwith the explosive device of my invention and perforations are formed inVthe tubular casing if they have not been already formed.

The structure of the well after the cement sheath has been fractured andperforations have been formed in the tubular casing is illustrateddiagrammatically in Figure 2. The structure comprises the casing 14having openings 18 therein and the surrounding cementitious sheath l5having a network of small fractures or cracks which provide passages foruids from the producing formation 11 into the openings in the casing.

To use a device of my invention in completing a well, it is lowered intothe well through the cemented casing to a level opposite the oil sand asshown in Figure l. The preferred embodiment of the device, as ilustrateddagrammatically in Figure 1, comprises an elongated inert core memberwhich can be, for example, a length of steel drill pipe. In thepreferred embodiment, the core member has a reduced diameter section 21,referred to in the claim as an explosive cord mounting section, which isadapted to receive wrappings of explosive cord. Advantageously thereduced diameter section is fcrmed by turning the cylindrical or tubularcore member on a lathe to cut away a section in which the explosive ccrdwrappings are protected from abrasion by the shouders v22 and 23 whichhave the original diameter of the core.

In the preferred embodiment of Figure 1, the core member is providedwith a shallow longitudinal groove 24 in the reduced diameter section2i. This groove is adapted to receive one or more strands of explo. ivecord which serve as detonating strands for the explosive cord which iswrapped about the core and presses tightly against the detonatingstrand.

In the preferred embodiment of the explosive device the wrappings ofexplosive cord are attached to the core member in the following manner.A detonating strand 25 is placed in the groove 24. To the free end ofstrand 2S is attached a detonating means, preferably an electricblasting cap, not shown in the drawing. The cap may be fired from groundlevel through an electrical conductor associated with the supportingcable 26 which is the means for lowering the device into a well.Alternatively the cap may be red by means of a go-devil dropped from theground level or by a time clock in known man` ner. The rest `of theexplcsive cord attached. t9 the ser@ member is wrapped as a coil orhelix 27 on the core and presses tightly against the longitudinaldetonating strand 2,5. The cord forming the helix Q7 can be either acontinuation of the strand of cord in groove-24 or a separate length ofexplosive cord. Y

The explosive device is advantageously provided with means for centeringthe device withfn a well. ligpre l shows such means in the form ofcentralizing strips 28 disposed at each end of the device.

An explosive cord which is particularly suitable for use in the deviceof the invention is known commercially yas Primacord. The cord is l/iinch in diameter and comprises an explosive core of PETN(pentaerythritol tetranitrate) and a water-proofed textile covering. Theexplosive rate is about 20,000 feet per second. This explosive cord istough, flexible and substantially impervious to water. lt is relativelyinsensitive to ordinary shock. friction, fire, or to temperatures met in-oil wells and therefore is safe to handle and resistant to prematureexplo- 4 sion when used in completing a well. lt is detonated by anelectric blasting cap. While Primacord is the preferred explosive cord,other explo1ive cords having the above-described properties can also beused for the explosive device of the invention.

In completing a well the explosive cord opposite the producing formation1l is detonated as by passing an electric current through a conductor incable 26 which detonates the blasting cap or other detonator (not shownin the drawing) on the explosive device and which initiates detonationof the explosive cord sections 25 and 27. The amount of explosive cordis predetermined by empirical tests and when this selected amount ofcord explodes within the casing 14, the resulting lateral force bulgesthe casing and applies a limited stress to the surrounding cement sheath15.

When the limited stress is applied by the explosion to the cement sheathwhile it is confined between the well casing and the wall of theproducing formation, many small fractures form in the portion of thesheath to which the stress is applied. However, the sheath retains itsgeneral outline as shown in Figure 2 and continues to support thesurrounding sand formation. Consequently, the shifting of sand whichresults in the clogging of conventional well strainers in oil sands isprevented or greatly reduced. The small fractures in the cement allowfluids to enter the Well from the producing formation but prevent theproduction of sand of any substantial particle size. The greatestpressure gradient during production from the resulting well structureoccurs as the uids pass through the fractures in the cement rather than,as in conventionally lcompleted wells, in the sand formation.Consequently, the forces tending to disturb the sand formation and causemovement of sand are considerably smaller than in conventional wellstructures.

Following the described fracturing operation, the tubular casing 14 canbe perforated if it has not Ialready been perforated. For this purpose aconventional casing perforator, for example, a mechanical perforator ofthe knife type, can be lowered into the well opposite the producingformation and the fractured cement sheath. The perforator is thenactuated to form holes in the casing which admit the uids filteringthrough the fractures in the cement sheath. The casing perforation canalso be performed before the fracturing operation if desired.

Although the structure illustrated in Figure l is preferred for theexplosive device `of the invention, the device can be modified and stillproduce advantageous results. For example, instead of being attached asspiral wrappings, the explosive cord can be attached longitudinally tothe elongated core `as a number of loops or strands, clipped orotherwise fastened along its length; It should be understood, however,that the structure of the explosive device illustrated in the drawing`is greatly preferred and has important advantages in Yits use in comfilel Dieting a well- For example the Provision Ot a lensitudinaldetonating strand under the spiral wrappings of explosive cord hasespecially proven to have imp rtant advantages. I have found that Withthis structure the possibility of misres with the device is considerablyless than when the longitudinal strand is omitted.

The explosive device of the invention can vary considerably in thelength and diameter of the co-re member, depending on such factors asthe inside diameter of the casing inthe producing zone and the top tobottom depth of the producing sand formation. A device of suitable sizefor many operations can be Vmade using as the core member a section of45/8 inches drill collar tubing of which a 10 foot length is turned on alathe to form an intermediate reduced diameter section for the Ywrappings of explosive cord. A shallow longitudinal the casing.

-channel is also milled for the detolnating strandf The length ofexplosive cord attached to the core per llinear foot must be sufficientto cause fraeturing of the cement sheath in the producing formation butinsufficient to cause rupturing of the tubular well casing. the amountof cord to be used will thus depend upon the thickness of the casingWall and the thickness of the cement sheath. As an example, using the 1Ainch diameter cord containing PETN, in 51/2 inch J-55 casing surrounded`by a 2 to 3 inch thick sheath of Portland cement, from about 5 to 2Ofeet of explosive cord per linear foot will fracture the cement withoutrupturnig Using a diiferent type of explosive cord or a differentthickness. of casing or cement sheath, the amount of cord to be usedmight differ considerably. However, bearing in mind the result desired,i. e., without rupturing the casing to form a fractured cement sheathwhich retains its general o-utline and supports the oil sand, the amountof explosive cord required can be found by routine experiments in a testho-le and the information gained used in an actual well completion.

I have Ycarried out particular operations which illustrate the resultsobtainable using an explosive device of my invention. 1n one suchoperation, to simulate the structure of a completed well in a producingformation, a cement sheath was formed about a length of slotted linerassembly. This was done by centering a length of 5 inch slotted liner ina length of 8% inch casing. The annular space was lled with cement andafter the cement had set the outer casing was removed from a l0 footsection of the assembly. The assembly, with cement exposed over a l0foot section, was then centered within a boiler flue and the cement-flueannulus was solidly packed with damp sand. The entire assembly waslowered into a test hole and an explosive device of my invention waslowered into the slotted liner. The explosive device had wrapped aboutit 6 feet of Primacord lper linear foot along a 32 inch length. ThePrimacord was detonated within the slotted liner, after which theassembly was raised to the surface and examined. It was observed thatthe cement opposite the 32 inch portion of the explosive device wasseverely fractured. The cement had both vertical and horizontalfractures, the vertical fractures on the average being about 2 inchesapart and the cross-cracking appearing, on the average, about every 6inches.

If it is found that the fractures formed in the cement by detonating theexplosive device of the invention are not large enough to produce fluidsas rapidly as desired, the fractures can be enlarged by any of a numberof known methods, for example, by treating the well with a chemicalagent such as hydrochloric acid.

Obviously many modifications and variations of the invention ashereinbefore set forth may be made without departing from the spirit andscope thereof and therefore only such limitations should beimposed asare indicated in the appended claim.

I claim:

An explosive device for fracturing the cement sheath surrounding a wellcasing which compn'sess an essentially rigid, vertically elongate,metallic core member of substantially circular transverse configurationthroughout its length, said core member having intermediate its endportions an explosive .cord mounting section of reduced diameter, eachof said end portions being provided with means for eentralizing the coremember in a well casing, said explosive cord mounting section having astraight longitudinal groove in its outer surface, a length of explosivecord, said explosive cord extending in one direction along the explosivecord mounting section and in the groove and thence extending along theexplosive cord mounting section in the opposite direction wrappedtightly as a helix about both the explosive cord mounting section andthe extent of the explosive cord disposed in the groove, the helicalextent of the explosive cord pressing tightly against the extent of theexplosive cord in the groove to afford a multiplicity of verticallyspaced con- References Cited in the tile of this patent UNITED STATESPATENTS 2,362,829 Kinley Nov. 14, 1944 2,414,349 Alexander Jan. 14, 19472,609,885 Silverman Sept. 9, 1952 2,650,539 Greene Sept. 1, 1953

